This invention relates to cleaning of hazardous waste formed in vacuum chambers of molecular beam epitaxy processing equipment during the process of growing layers of Indium Phosphide material on a substrate, and, more particularly, to a method and vacuum cleaning apparatus for removing hazardous residue from the vacuum chamber without endangering either the maintenance personnel or the environment.
The molecular beam epitaxy (EMBED) process has been accepted as a preferred fabrication process for the production of Indium Phosphide (xe2x80x9cInPxe2x80x9d) type heterojunction bipolar transistors (xe2x80x9cHBTxe2x80x9d) and high electron mobility transistors (xe2x80x9cHEMTxe2x80x9d). In that process selected group III and group V elements (of the periodic table of elements) are heated to vaporization temperatures in respective shuttered effusion cells inside of a vacuum chamber for deposit on a substrate of InP placed within that chamber. The elements that are vaporized at different times typically include Indium, Gallium, arsenic, silicon and phosphorous. As each element is needed in the fabrication process, a shutter covering the respective heated element, is opened, allowing the vapor of the element, metal or non-metal, to form a beam of molecules of the element that exits the associated effusion cell and coats the internal apparatus of the vacuum chamber. Some of that evaporated material deposits on the InP substrate to build-up epitaxial layers of the material on the substrate, which is the purpose of the process. By appropriate control of the effusion cell shutters, the desired layered structure that defines the transistor is ultimately produced. Typically, the MBE deposition process continues for long periods and, typically, takes between four and six months of continuous operation to complete a respective HBT or HEMT growth campaign. The remainder of the evaporated material during the processing, however, remains on the walls of the vacuum chamber, and constitutes the waste or, as otherwise termed, the residue of the MBE process. The present invention concerns safely cleaning up that waste.
Eventually with continued processing, the chemical elements within the effusion cells in the MBE vacuum chamber are depleted, and must be replenished. At that time the heat is turned off, the vacuum chamber is vented to the atmosphere, the chamber port is removed to allow full access to the internal apparatus. The inner walls of the chamber are covered with the residue of the processing, the portion of the material that deposited on the walls of the chamber. The residue on the chamber wall may comprise chunks or particulate of any and all of the materials vaporized during processing. The predominant ingredients of that residue are particulate arsenic and red phosphorous material.
Before restocking and reusing the vacuum chamber, the chamber must be cleaned of that residue. To do so, using a water-scraper maintenance personnel scrape the walls or use an abrasive tool to remove the residue. The material scraped off the chamber walls falls to the bottom of the chamber and to other regions of the chamber that are difficult to reach, where such debris collects and must be removed.
The volatile nature of the red phosphorus in that debris poses a problem in the cleaning procedure. In other MBE processing systems in which Arsenic is the material that is evaporated during processing, and the residue of the process contains essentially only Arsenic particulate that is scraped from the chamber walls, cleaning up of the Arsenic residue from the bottom of the chamber is quite simple. A HEPA vapor vacuum cleaner is used to vacuum up the particulate. The HEPA filters in the vacuum cleaner prevent arsenic particulate from escaping to the environment so that maintenance personnel are not exposed to a hazard. One known HEPA vapor vacuum cleaner is the Vapor Vac(trademark) model vacuum cleaner marketed by Nilfisk of America. Such a convenient approach to cleaning the chamber is not possible when red phosphorus is involved.
Red phosphorus, a metal of an allotropic form referred to as P-2, has quite different chemical properties than arsenic, and different even from the white form of phosphorus (e.g. P-4 or white phosphorus). Red phosphorus is easily ignited by friction, and will burst into flame. Even when the flame is extinguished, the phosphorus may continue to smolder, posing a continuing danger. Not only posing a fire hazard, one of the products of combustion of the red phosphorus when ignited in air is phosphorus pentoxide, a very corrosive gas.
Due to the foregoing volatility and high risk of ignition of the red phosphorus, the described scraping procedure for cleaning poses risks for cleaning personnel, who must take special safety precautions, including wearing fire. resistant apparel and breathing apparatus in cleaning the chamber. Further, due to that volatility and high risk of ignition of the red phosphorus, a standard HEPA/Vapor vacuum cleaner is not recommended for use with red phosphorous material (and, when queried, the manufacturer advised against such use). Even vacuum cleaners advertised as xe2x80x9cexplosion proofxe2x80x9d, such as the EXP 1-75 vacuum cleaner manufactured by Nilfisk of America or the accessories from the SCV spray cleaner vacuum from that company, were, on advice of the manufacturer, said to be incapable of handling and were not designed to capture Red Phosphorus or other flammable metals. As an advantage, the present invention provides a way of safely cleaning the MBE vacuum chamber of residue containing red phosphorus or like volatile material.
Accordingly, a principal object of the invention is to improve the technique for cleaning MBE vacuum chambers.
A further object of the invention is to provide a system for safely vacuuming up particulate red phosphorus material or any other volatile flammable metal particulate having like combustion properties and reaction to that of Red Phosphorus.
And a still further object of the invention is to provide a method of safely removing hazardous residue that contains red phosphorus from the vacuum chamber of MBE processing equipment.
In accordance with the foregoing objects and advantages, the invention is characterized by maintaining the red phosphorus residue wetted with water during the cleaning of the vacuum chamber. A vacuum cleaner is used to extract the particulate scraped from the chamber walls, while, simultaneously, water is sprayed onto those walls. Particulate and water drops to the bottom of the chamber and with the residue forms a slurry that is drawn up by the negative pressure produced by the vacuum cleaner. The red phosphorus is maintained wet even after being drawn into the vacuum. As a additional aspect to the invention, the water is de-ionized water.
A vacuum cleaner in accordance with the invention includes a water spray carried on the vacuum hose nozzle for spraying water out while simultaneously vacuuming up the slurry of residue and water for deposit within a canister. A vacuum cleaner constructed in accordance with the invention further includes a flexible expandable (and contractible) hose internal to the canister of the vacuum cleaner. The flexible hose extends to the canister inlet while the other end of the hose, coupled to a float, is held above the level of any slurry drawn into the canister during vacuuming.
The foregoing and additional objects and advantages of the invention together with the structure characteristic thereof, which was only briefly summarized in the foregoing passages, will become more apparent to those skilled in the art upon reading the detailed description of a preferred embodiment of the invention, which follows in this specification, taken together with the illustrations thereof presented in the accompanying drawings.